This invention relates to compounds effective as contrast agents in diagnostic imaging. In one aspect, this invention relates to magnetic resonance imaging (MRI) of human or non-human animal subjects using metal complexes of substituted nitrogen-containing fifteen-membered macrocyclic ligands which are conjugated to a targeting biomecule as contrast agents. In another aspect, this invention relates to manganese(II) complexes of substituted nitrogen-containing fifteen-membered macrocyclic ligands which are conjugated to a targeting biomolecule as MRI contrast agents.
X-rays have long been used to produce images of human and non-human animal tissue, e.g. the internal organs of a patient, the patient being positioned between a source of X-rays and a film sensitive to the rays. Where organs interfere with the passage of the rays, the film is less exposed and the resulting developed film is indicative of the state of the organ.
More recently, nuclear magnetic resonance (NMR) has been developed as an imaging technique, i.e. MRI. MRI avoids the harmful effects sometimes attending X-ray exposure. For improved imaging with X-rays, patients have been given enhancers prior to imaging, either orally or parenterally. After a predetermined time interval for distribution of the enhancer through the patient, the image is taken. To obtain a good image it is desirable that the time after the taking of enhancer be kept to a minimum. On the other hand there is a decrease in effectiveness with time, so desirably the decay should be relatively slow so as to provide a substantial time interval during which imaging can be done.
In the NMR imaging process, protons in the water of the body relax via two mechanisms. The respective relaxation times are referred to as T1 and T2. The rate at which the relaxation process occurs may be altered for some water molecules by giving values that contrast with the norm.
Compounds that enhance NMR images, referred to as contrast agents, are generally paramagnetic in nature. These may be organic free radicals or transition/lanthanide metals which have from one to seven unpaired electrons.
A necessary prerequisite of any ligand that binds a metal to form a contrast agent is that the resulting contrast agent be stable so as to prevent the loss of the metal and its subsequent accumulation in the body. Other considerations include an ability to reversibly bind water, which in turn increases it contrastability and decreases the dose level required. This ability is clearly important since the interaction between any two nuclear spins through space decreases at a rate equal to the reciprocal of the distance raised to the sixth power.
U.S. Pat. No. 4,647,447 discloses use of an NMR image enhancer consisting of the salt of an anion of a complexing acid and a paramagnetic metal anion. A preferred embodiment is the gadolinium chelate of diethylenetriaminepentaacetic acid (Gd DTPA), which is now commercially available from Nycomed Salutar, Inc. under the trade name Magnevist for use as an NMR contrast agent. From the data reported therein these appear to perform well. However, this compound is rapidly excreted by the kidneys, making the timing of the injection extremely critical. Furthermore, there is virtually no uptake by any solid-organ, such as the heart, pancreas or liver.
However, while a number of gadolinium contrast agents are known, there remains the possibility that small amounts of free lanthanides are being released, by decomposition of the agent, into the body. Not being a naturally existing metal in the body, little is known about long term effects.
Other nitrogen-containing macrocyclic ligands have been suggested for use as NMR contrast agents. Jackels, S. C. et al, “Aqueous Proton NMR Relaxation Enhancements by Manganese(II) Macrocyclic Complexes: Structure-Relaxivity Relationships”, Inorg. Chem., 31, 234-239 (1992) discloses fifteen-membered nitrogen-containing ring complexes. However, these compounds suffer from being insufficiently stable and/or colored, and as such are inadequate for application as MRI contrast agents.
Therefore, it would be highly desirable to develop alternative contrast agents which avoid one or more of the aforementioned disadvantages. It would also be desirable to be able to direct the contrast agents to a desired target in the body where the compound can be concentrated for optimal effect. Without some way to render the compounds “targeting”, increased dosages are sometimes necessary in order to obtain an efficacious concentration at the site of interest. Such increased dosages can sometimes result in undesirable side effects in the patient.
It has now been discovered that metal complexes of substituted nitrogen-containing macrocyclic ligands which have increased kinetic, thermodynamic and oxidative stability, and which can be substituted to control lipophilicity, i.e. biodistribution, avoid the problems of the aforementioned contrast agents while providing good contrastability. It has also now been found that the macrocycles or metal complexes of the present invention can be attached, i.e. conjugated, to one or more targeting biomolecule(s) via a linker group to form a targeting biomolecule-macrocycle or targeting biomolecule-metal complex conjugate.